1. Field of the Invention
The present invention relates to a copier, printer, plotter, facsimile apparatus or similar electrophotographic image forming apparatus and more particularly to a developing method using a dry two-ingredient type developer and a device for practicing the same.
2. Description of the Background Art
An image forming apparatus, in many cases, uses a magnet brush type developing device operable with a dry two-ingredient type developer, i.e., a mixture of magnetic carrier grains and toner grains. This type of developing device usually includes a rotatable sleeve in which a magnet roller provided with a plurality of magnetic poles is disposed. The magnetic carrier grains on which the toner grains are deposited are magnetically retained on the sleeve and conveyed to a developing zone thereby.
More specifically, the toner grains and carrier grains are agitated together and charged to opposite polarities by friction. In the developing zone, the toner grains thus charged are transferred to a latent image opposite in polarity to the toner grains, thereby producing a corresponding toner image. The toner grains are charged most by a doctor blade or metering member that rubs the toner grains and carrier grains in a small gap. However, the doctor blade causes stress to act on the toner grains at the same time as it charges them by friction. The stress causes an outside substance covering the individual toner grain to be buried in the toner grain, thereby lowering the fluidity of the toner. Consequently, the charge distribution of the toner grains becomes irregular and brings about various image defects including background contamination as well as toner scattering.
In light of the above, Japanese Patent Laid-Open Publication Nos. 5-119518 and 11-143115, for example, propose to adjust the composition of the outside substance, which covers the toner grains, such that the matter is buried little in toner grains despite the stress mentioned above. Also, Japanese Patent Laid-Open Publication No. 8-54750 proposes a specific grain size and a specific viscoelasticity of toner grains.
However, none of the above proposals is a versatile measure for various kinds of developing devices because they cope with the stress from the toner grain side. More specifically, although the expected effect may be achieved with some kind of developing device, it cannot be fully achieved with, e.g., a developing device of the kind exerting heavier stress or a small-size developing device storing only a small amount of developer.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 10-228175, 2000-267337 and 2000-347442.
It is an object of the present invention to provide a developing method capable of reducing the burying of an outside substance in toner grains without regard to the kind of a developing device even when the toner grains are agitated over a long time, thereby insuring high image quality, and a device for practicing the same.
It is another object of the present invention to provide an image forming apparatus using the above developing device for thereby maintaining image quality high.
A developing method of the present invention develops a latent image formed on an image carrier with a developing device including at least a developer carrier and a doctor blade. The developer carrier magnetically causes a two-ingredient type developer made up of toner grains and magnetic carrier grains to deposit thereon and conveys it to a developing zone where the developer carrier faces the image carrier. The doctor blade regulates the thickness of the developer forming a layer on the developer carrier. The ratio of a dynamic torque W (kgfxc2x7cm) to act on the developer while the developing device is in operation to an amount of toner grains M (g) contained in the developer is selected to be 4.0xc3x9710xe2x88x922 or below.
A developing device for practicing the above method and an image forming apparatus including the developing device are also disclosed.